The invention relates to a photopolymer formulation comprising matrix polymers, writing monomers and photoinitiators, the use of the photopolymer formulation for the production of optical elements, in particular for the production of holographic elements and images, a method for the exposure of holographic media comprising the photopolymer formulation and special fluorourethanes.
WO 2008/125229 A1 describes photopolymer formulations of the type mentioned at the outset. These comprise polyurethane-based matrix polymers, acrylate-based writing monomers and photoinitiators. In the cured state, the writing monomers and the photoinitiators are embedded with spatial distribution in the polyurethane matrix. It is also known from the WO document that dibutyl phthalate, a conventional plasticizer for industrial plastics, can be added to the photopolymer formulation.
For the uses of photopolymer formulations in the fields of use described below, the refractive index modulation Δn produced by the holographic exposure in the photopolymer plays the decisive role. During the holographic exposure, the interference field of signal light beam and reference light beam (in the simplest case, that of two plane waves) is mapped into a refractive index grating by the local photopolymerization of, for example, highly refracting acrylates at sites of high intensity in the interference field. The refractive index grating in the photopolymer (the hologram) contains all information of the signal light beam. By illuminating the hologram only with the reference light beam, the signal can then be reconstructed. The strength of the signal thus reconstructed in relation to the strength of the incident reference light is referred to as diffraction efficiency, DE below. In the simplest case of a hologram which forms from the superposition of two plane waves, the DE is obtained from the quotient of the intensity of the light diffracted on reconstruction and the sum of the intensities of the incident reference light and diffracted light. The higher the DE, the more efficient is a hologram with respect to the necessary quantity of reference light which is required for making the signal visible with a fixed brightness. Highly refracting acrylates are capable of producing refractive index gratings having a high amplitude between regions having the lowest refractive index and regions having the highest refractive index and hence permitting holograms with high DE and high Δn in photopolymer formulations. It should be noted that the DE is dependent on the product of Δn and the photopolymer layer thickness d. The greater the product, the greater is the possible DE (for reflection holograms). The width of the angular range in which the hologram is visible (reconstructed), for example on monochromatic illumination, depends only on the layer thickness d. On illumination of the hologram with, for example, white light, the width of the spectral range which can contribute to the reconstruction of the hologram likewise depends only on the layer thickness d. The smaller d in this case, the greater are the respective acceptance widths. If it is therefore intended to produce bright and readily visible holograms, a high Δn·d and a small thickness d are desirable, in particular so that DE will be as large as possible. This means that the higher Δn, the more latitude is achieved for producing bright holograms by adaptation of d and without loss of DE. The optimization of Δn is therefore of outstanding importance in the optimization of photopolymer formulations (P. Hariharan, Optical Holography, 2nd Edition, Cambridge University Press, 1996.).